In semiconductor manufacturing, contact plugs are used to connect conducting layers on two different levels. FIGS. 1 and 2 show cross-sectional views of a semiconductor substrate 101, illustrating the steps of a conventional method for forming a contact plug. The term substrate is understood to possibly include a semiconductor wafer, active and passive devices formed within the wafer, and layers formed on the wafer's surface.
Formed atop of the substrate 101 are metal lines 103. Typically, the metal lines 103 are a composite stack of different materials. The primary sandwiched material is aluminum, copper, or an aluminum-copper alloy. The tops and bottoms of the metal lines 103 may be covered with a barrier layer as is conventional in the art.
Still referring to FIG. 1, an insulating layer 105 is deposited atop the substrate 101 and the metal lines 103. The insulating layer 105 is commonly referred to as an intermetal dielectric. Next, a contact opening 109 is formed by etching through the insulating layer 105 using conventional photolithography techniques using a photoresist layer 107. Note that in FIG. 1, the contact opening 109 does not land squarely atop of the metal line 103. This is a result of misalignment in the photolithography process and is not uncommon, particularly as device geometry is further reduced.
Next, turning to FIG. 2, a barrier metal layer 201 is deposited into the contact opening 109 and atop the insulating layer 105. Typically, the barrier metal layer 201 is a titanium nitride material. Then, a conducting layer 203 is deposited into the contact opening 109 and atop the barrier metal layer 201. The conducting layer 203 is often formed from the chemical vapor deposition (CVD) of tungsten.
To complete the process, the portion of the conducting layer 203 atop the insulating layer 105 and the portion of the barrier metal layer 201 atop the insulating layer 105 are removed by chemical mechanical polishing (CMP). The portion of the conducting layer 203 inside the contact opening 109 remains to form the contact plug.
Note that one result of the misalignment during the photolithography step is that the contact plug extends down the side of the metal line 103 as highlighted by area 205. With the materials described above, the flouride gas from the WF.sub.6 used in the deposition process of the conductive layer 203 will attack the aluminum in the metal line 103 if weak spots are present in the titanium nitride barrier metal layer 201. This will reduce the integrity of the contact plug, as well as cause other detrimental effects.
What is needed is an improved method for forming a contact plug without the problems of the prior art noted above.